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An inductance-capacitance-resistor (LCR) loop is composed, by connecting an inductive coil with a capacitance element end by end. Without any electric connection with other circuits, higher quality factor can be easily obtained. The loop is inserted into the electromagnetic coupling path of coil-based sensing system, such as the case of eddy-current sensor. When the system is working at the natural resonant frequency of the loop, the coupling efficiency and, therefore, the sensing performance could be improved significantly. Protocol systems, simulating the operation cases of wireless passive sensor and eddy current proximity sensor, are constructed and tested experimentally. The results indicate that, in case of coil-coupled wireless passive sensing system, the received signal's amplitude is enlarged remarkably with a coaxially inserted LCR loop. Moreover, when the loop is coaxially placed on top of the sensing coil of eddy current sensor, not only higher sensitivity and resolution, but also larger monotonic detection range could be achieved.